Age of Enlightenment

the marriage of optics and genetics: a promising young union // that controls the firing of particular neurons // which have been reengineered to be activated by light // uncovering the brain’s mysteries with a flash Age of Enlightenment by cathryn delude // photographs by sam Kaplan uch of what science knows about the human brain has come through deduc- 13 M tion. If a stroke or trauma has destroyed a particular area, researchers can look at what that person can no longer do—talk, move the left pinky, do math—and infer that the affected region is linked to that behavior. In animal models, researchers often produce lesions artificially, or they inject a drug to inhibit or excite neural activity in a specific area. Yet as important as this approach has been, there are many things it can’t accomplish. Chief among those is pin- pointing which of the many kinds of cells in a given brain region are the ones that matter. As a result, when it comes to autism, Alzheimer’s disease and a long list of mental illnesses, what we do understand is dwarfed by all that we can only imagine. Treatments, too, are often a matter of trial and error. To try to prevent intractable epilep- tic seizures, for example, surgeons may destroy a part of the brain they believe is implicated. Often it works; sometimes it doesn’t. Understanding aberrant behavior, obsessive thoughts, learning disabilities, depression, anxiety, aggression—for all of those, the learning curve remains very steep. Much of the problem stems from the brain’s sheer com- plexity. “It’s made of 100 billion interconnected cells, which fall into many distinct classes—differing by shape, molecular composition and function—and which change in different ways in different brain disorders,” says Edward Boyden, a neuroscientist and biological engineer in the MIT Media Lab, whose goal is to develop technologies for “fixing the broken brain.” But it’s not just the number of neurons that makes the brain so challenging; it’s also that they fire in essentially arbi- trary numbers of patterns depending on what we are doing, perceiving and thinking, how we feel emotionally and wheth- er we are well or sick. Moreover, some neurons excite other neurons, some inhibit others, and some fine-tune how sensi- Age of Enlightenment tive other neurons are to being excited or inhibited. Neurons within the same brain region may connect to others locally or Spring 13 // protomag.com use long arms called axons to reach remote parts of the brain. Further complicating attempts to make sense of all this, diverse types of neurons are likely to be tightly intermingled in tiny volumes of brain tissue. In 1979, Francis Crick, who helped discover the structure of DNA in the 1950s and later turned to neuroscience, cau- tioned that scientists could never understand the brain unless they could precisely control discrete sets of neurons to see what they do. “Without that level of understanding, I re- contains both a light-absorbing component—retinal, a vitamin alized we cannot fully help patients with depression, schizo- A derivative—and a channel pore that is closed when the opsin is phrenia and autism,” says Karl Deisseroth, a psychiatrist and not illuminated. When the opsin’s retinal absorbs light, the pro- neuroscience researcher at Stanford University. tein changes form and temporarily opens up a pore that lets ions But a new technology, optogenetics, which Deisseroth and through. In effect, these naturally occurring opsins convert light Boyden helped develop at Stanford, along with Feng Zhang, a into changes in cellular voltage. And because a voltage change is graduate student in Deisseroth’s lab, and two other research- what causes neurons to fire, in 2000 Deisseroth and Boyden be- ers from Germany, Georg Nagel and Ernst Bamberg, could gan brainstorming ways they might harness the special proteins finally begin to map the brain’s vast frontiers. Combining op- to control neurons. In 2004, the team decided to focus on Chan- tics and genetics, optogenetics allows scientists to control in- nelrhodopsin-2 (ChR2), a protein from a one-celled alga that has dividual classes of neurons distributed among many other cell an “eye” to guide the organism toward the daylight at the sur- types—and with a flash of light. face of a murky pond for photosynthesis. When exposed to blue Introduced in 2005, optogenetics remains a young technol- light, a pore in the cell opens and positively charged ions flow in. ogy. Yet like a child prodigy, it has garnered rave reviews. It has Deisseroth and Zhang, who is now a professor at MIT, used been called transformative, exquisite and a great leap forward. genetic engineering techniques to deliver the gene for ChR2 Researchers around the world are using it to illuminate—liter- into the nucleus of rodent neurons they grew in cell cultures; ally and figuratively—the mysterious brain and to investigate the neurons then successfully produced the opsin. Boyden, links between brain circuits and specific behaviors with a preci- who has an MIT degree in engineering and physics, rigged a sion that could only have been imagined less than a decade ago. system for directing blue light on those novel neurons. Amazingly, the neurons fired instantly when illuminated. or years before optogenetics was invented, reports had Also wonderful: The neurons immediately returned to their circulated about microorganisms that produce proteins normal state when the light pulsed off, giving an unprec- called opsins. An opsin spans the cell membrane and edented precision in timing neural activity. The process was F perfectly simple, requiring only a single gene. The team published its work in August 2005 in Nature Neu- roscience, and they and other scientists adapted the system to try in living mice. Researchers insert the gene for ChR2 into protomag.com // Spring 13 a harmless virus, which is injected into a region of the mouse’s brain and “infects” the neurons there with the gene. The neurons then produce the light-activated protein all over their cell membranes. Next, implanted optic wires threaded to that brain region direct light there. When the light pulses on, only those cells bearing the opsin fire. One researcher’s By 2007, Boyden—who by then had his own lab at MIT— Zhang, and Deisseroth independently showed that another optogenetics and naturally occurring opsin, halorhodopsin, responds to yellow 15 or green light and inhibits neurons, preventing them from electrophysiology studies firing. Halorhodopsin allows researchers to zero in on particular kinds of neurons and control when they fire—in live, in mice show that neurons conscious animals. “People had been dreaming of this ability for decades,” says Wim Vanduffel, a neurophysiologist and ra- in aggression and mating diologist at Massachusetts General Hospital who has begun are intermingled. using the technology in his research. Optogenetics has clear advantages over electrophysiology, a technology that uses implanted electrodes to force neurons to fire. But a combination of the technologies can be particularly impulses from neurons in the hypothalamus as male mice powerful. Electrophysiology can record how neurons fire both fought potential male competitors and as they mated with during normal activity and when optogenetically controlled. females. “We were surprised that the nerve cells involved in David Anderson, a neurobiologist at the California Institute both aggression and sex are closely intermingled in the same of Technology, used this approach to explore aggression and neighborhood,” Anderson says. sex in mice. With electrophysiology, he recorded electrical Normally, within a split second of an intruder male mouse entering a cage, the resident male streaks toward it and bites its neck. But the resident mouse will not attack a female—or a castrated male. When Anderson optogenetically activated neurons in the hypothalamus, however, the male would attack anything: a castrated male, an inflated rubber glove and even a female, unless he was in the midst of sex; then he wouldn’t go after her until after climaxing. Anderson’s studies show not only that neurons involved in aggression and mating are intermingled, but also that interactions with females inhibit the neurons normally active in male aggression. “If this holds for humans, perhaps we’ll learn that something goes wrong with this inhibition in sexual pathologies when circuits get miswired,” he says. He’s plan- ning experiments to trace that circuitry. 16 arl Deisseroth believes optogenetics is well suited to studying neuropsychiatric disorders, and many of K his experiments in rodents aim to parse different neural circuits that account for varied aspects of depression and its diverse causes. Recently, he and his collaborators have concentrated on learning how neurons in one brain re- not have the anticipated effect on anxious behavior—likely gion may connect to others within the same region and also because the neurons project along pathways that can have op- to other regions. posing functions and counteract the anxiety circuit. Tye want- For example, Deisseroth and Kay Tye, a principal inves- ed to selectively activate or inhibit just the BLA cells that pro- tigator in neuroscience at MIT, who was previously a post- jected to the CeA, so she exploited the fact that when neurons doctoral researcher in Deisseroth’s lab, wanted to manipulate express the gene for an opsin, the light-activated protein ap- a “microcircuit” of connections within the amygdala, which pears all over the neuron’s membrane, including on the axon numerous previous studies have implicated in stress and anxi- fibers that connect to other neurons. TheBLA axons reaching ety. They wanted to control signals running from the basal lat- the CeA had the light-activated proteins, but none of the neu- eral amygdala (BLA) to the central amygdala (CeA), and see rons residing in the CeA had them.

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